Java on the Mac installs in such a way that Eclipse can't mount individual JRE library jars as they appear within a bundle.
To add an individual jar to a project build path you will need to manually edit the libaryInfos.xml file:
cd workspace/.metadata/.plugins/org.eclipse.jdt.launching/
nano libraryInfos.xml
Now add the jar you want to the bootpath element:
<bootpath>
#ADD PATH TO NEW JRE LIBS HERE
<entry path="/Library/Java/JavaVirtualMachines/jdk1.7.0_07.jdk/Contents/Home/jre/lib/resources.jar"/>
<entry path="/Library/Java/JavaVirtualMachines/jdk1.7.0_07.jdk/Contents/Home/jre/lib/rt.jar"/>
<entry path="/Library/Java/JavaVirtualMachines/jdk1.7.0_07.jdk/Contents/Home/jre/lib/sunrsasign.jar"/>
<entry path="/Library/Java/JavaVirtualMachines/jdk1.7.0_07.jdk/Contents/Home/jre/lib/jsse.jar"/>
<entry path="/Library/Java/JavaVirtualMachines/jdk1.7.0_07.jdk/Contents/Home/jre/lib/jce.jar"/>
<entry path="/Library/Java/JavaVirtualMachines/jdk1.7.0_07.jdk/Contents/Home/jre/lib/charsets.jar"/>
<entry path="/Library/Java/JavaVirtualMachines/jdk1.7.0_07.jdk/Contents/Home/jre/lib/jfr.jar"/>
<entry path="/Library/Java/JavaVirtualMachines/jdk1.7.0_07.jdk/Contents/Home/jre/lib/JObjC.jar"/>
<entry path="/Library/Java/JavaVirtualMachines/jdk1.7.0_07.jdk/Contents/Home/jre/classes"/>
</bootpath>